BEIJING, China – Choking smog all but shut down one of northeastern China’s largest cities on Monday, forcing schools to suspended classes, snarling traffic and closing the airport, in the country’s first major air pollution crisis of the winter.

An index measuring PM2.5, or particulate matter with a diameter of 2.5 micrometers (PM2.5), reached a reading of 1,000 in some parts of Harbin, the gritty capital of northeastern Heilongjiang province and home to some 11 million people.

A level above 300 is considered hazardous, while the World Health Organization recommends a daily level of no more than 20.